1. Field of the Invention
The present invention relates to a balance, a timepiece movement having the balance, a timepiece and a manufacturing method of the balance.
2. Background Art
A speed regulator for a mechanical timepiece is generally configured to have a balance and a hair spring. Such a balance includes a balance staff and a balance wheel fixed to the balance staff. The balance is a member which oscillates by cyclically rotating forward and backward around an axle of the balance staff. In this case, it is important that an oscillation cycle of the balance is set to be within a predetermined control value. This is because a rate of the mechanical timepiece (degree indicating whether the timepiece is fast or slow) varies if the oscillation cycle is beyond the control value. However, the oscillation cycle is likely to vary due to various causes, and for example, also varies due to a temperature change.
Here, an oscillation cycle T described above is expressed by Equation 1 as follows.
                    T        =                  2          ⁢                                          ⁢          π          ⁢                                    I              K                                                          Equation        ⁢                                  ⁢        1            
In Equation 1, the “moment of inertia of the balance” is indicated by I and a “spring constant of the hair spring” is indicated by K. Therefore, if the moment of inertia of the balance or the spring constant of the hair spring varies, the oscillation cycle also varies.
Here, a metal material used in the balance includes a material whose linear expansion coefficient is generally positive and which is expanded due to a temperature rise. Therefore, the balance wheel is radially enlarged to increase the moment of inertia. In addition, since the Young's modulus of a steel material which is generally used in the hair spring has a negative temperature coefficient, the temperature rise causes the spring constant to be lowered.
As described above, in a case of the temperature rise, the moment of inertia is increased accordingly and a spring coefficient of the hair spring is lowered. Therefore, as is apparent from Equation 1 expressed above, the oscillation cycle of the balance has characteristics of being shorter at a low temperature and being longer at a high temperature. For that reason, as temperature characteristics of the timepiece, the timepiece is fast at the low temperature and slow at the high temperature.
Therefore, as a measure to improve the temperature characteristics of the oscillation cycle of the balance, the following two methods have been known.
The first method is a method where the temperature coefficient of the Young's modulus near an operating temperature range of the timepiece (for example, 23° C.±15° C.) is caused to have positive characteristics by employing a constant elastic material such as so-called Coelinvar as the material of the hair spring. In this manner, in the operating temperature range, it is possible to cancel the change in the moment of inertia of the balance with respect to the temperature, thereby enabling temperature dependence of the oscillation cycle of the balance to be lessened.
As the second method, there has been a known method of using a bimetal where metal plates formed of materials having different thermal expansion coefficients are radially bonded together in a portion of multiple rim portions configuring the balance wheel, while one end portion in a circumferential direction is set to be a fixed end and the other end in the circumferential direction is set to be a free end (refer to “The Theory of Horology” published by Swiss Federation of Technical Colleges, English Version, Second Edition, April 2003, pages 136 to 137).
Out of the bimetals, for example, the material of the metal plate positioned radially inward employs a low thermal expansion material such as Invar and the material of the plate positioned radially outward employs a high thermal expansion material such as brass. In this manner, in the case of the temperature rise, the bimetals are deformed inward so as to move the free end side radially inward due to a difference in the thermal expansion coefficients. This enables an average diameter of a rim portion to be radially reduced and enables the moment of inertia to be lowered. Thus, it is possible to cause the temperature characteristics of the moment of inertia to have a negative slope. As a result, it is possible to lessen the temperature dependence of the oscillation cycle of the balance.
However, in the above-described first method, there is a possibility that when manufacturing the hair spring using the constant elastic material such as the Coelinvar, the temperature coefficient of the Young's modulus may vary greatly depending on composition during a melting process and various processing conditions during a heat treatment process. Therefore, a strict manufacturing control process is required, thereby not facilitating the production of the hair spring. Accordingly, in some cases, it is difficult to cause the temperature coefficient of the Young's modulus to be positive near the operating temperature range of the timepiece.
In addition, in the above-described second method, as a general method for configuring the balance wheel, after brazing an annular metal member formed of the high expansion material around an outer periphery of a metal member which is positioned radially inward and formed of the low expansion material by using a brazing filler metal, the balance wheel is formed through a cutting process by turning. Accordingly, an amount of the brazing filler metal is not constant depending on a size of a clearance between parts, and there are large variations in the moment of inertia when the balance wheel is formed. In addition, radial deviation between the parts is likely to occur and a ratio of a plate thickness of a low thermal expansion portion and a plate thickness of a high thermal expansion portion is not constant in multiple rim portions when the balance wheel is formed. Thus, there is a problem in that a deformation volume of the free end has large variations due to the temperature change. In addition, as another method for configuring the balance wheel, an annular high expansion material having a lower melting point than a low expansion material is arranged outside the low expansion material finished to have a predetermined outer diameter, the high expansion material is bonded to the low expansion material by heating these materials at a temperature for melting the high expansion material only, and then the balance wheel is formed through the cutting process by turning. In this method, since the brazing filler metal is not interposed between the low expansion material and the high expansion material, there is no possibility that the moment of inertia may have large variations. However, when forming the balance wheel, inner and outer diameter processing for the low expansion material and outer diameter processing for the high expansion material are processes separate from each other. Thus, it is difficult to keep a constant ratio of plate pressures of respective materials, thereby causing a problem in that the deformation volume of the free end has the large variations due to the temperature change. Furthermore, in both of the manufacturing methods, it is necessary to heat the brazing filler metal or the high expansion material at a high temperature of 800° C. or higher for example, thereby leaving a large residual stress because of a difference in the linear expansion coefficient of the materials during a cooling process. In addition, since it is necessary to perform processing after bonding, a processing stress is left on the balance wheel. Therefore, the deformation is likely to occur when forming the free end at a portion of the rim, and the deformation due to a time-dependent change is likely to occur, thereby causing a problem in that a balance of the moment of inertia tends to deteriorate. As described above, there is a problem in that a target value of the moment of inertia which has been set when designing is largely deviated, and further, a rotation balance deteriorates due to the temperature change. Therefore, it is necessary to adjust the moment of inertia for the overall balance or to adjust the deformation volume for the respective rims with respect to the temperature. In practice, it is necessary to carry out work for attaching a plurality of balance screws to the rim portion and adjusting an attachment position of the balance screws or screwing intensity. For example, even if the temperature rises, if the timepiece is slow, a process of correcting the moment of inertia is performed by carrying out the work such as changing work to transfer the balance screws to the free end side.
As described above, since fine adjustment work using the balance screws is required in practice, the temperature correction needs labor and time, thereby resulting in poor workability.